During the production of a hydrocarbon well, it is necessary to monitor the relative volumetric flow rates of the different phases (e.g. oil, gas and water) of the multiphase fluid mixture flowing into the pipe of the well from the hydrocarbon bearing zones. Further, current hydrocarbon well often comprises vertical well section, inclined well sections and horizontal well sections. The interpretation of the flow in such complex wells is challenging because small changes in the well inclination and the flow regime influence the flow profile. Thus, an accurate monitoring requires sensors or probes capable of imaging a surface section or a volume section of the pipe and providing an estimation of the surface section or the volume section occupied by each phase.
Several sensors or probes are known in the art, for example gradiomanometer, capacitance sensors, imaging instruments comprising arrays of sensors (based on radio-frequency, X-Rays, ultrasonics, etc. . . . ), and local probes.
The document U.S. Pat. No. 6,023,340 describes single point optical probes for measuring three-phase characteristics of fluid flow in a hydrocarbon well and methods of processing signals generated by the probe. A probe having a single fiber optic is coupled to a light source and apparatus for detecting reflectance and fluorescence. Light is delivered to the tip of the probe where it either is internally reflected in the probe or exits the probe and illuminates the fluid ambient the probe tip. If the fluid at the probe tip is oil, the light exits the probe, illuminates the oil, and causes the oil to fluoresce. If the fluid is water, no fluorescence occurs. If the fluid is gas, at least some light is internally reflected in the probe. A detection system including at least one beam splitter and fluorescence and reflectance detectors is provided in conjunction with the probe. Preferably, the fluorescence detector is coupled to the fiber optic by a wavelength division multiplexer. A preferred signal processing system for detecting oil, gas, and water provides two quasi-binary indicators: gas/liquid and oil/not oil. Three of the four possible indications (gas-not oil, liquid-not oil, and liquid-oil) give results indicating whether the fluid at the probe tip is gas, water, or oil.
This optical probe comprising a single fiber results in a complex optical system that includes at least one directional coupler and a wavelength division multiplexer, or two directional couplers. As consequence, this complex optical system is difficult to integrate into a downhole measuring tool, e.g. a logging tool.
The optical analysis probe and the downhole measuring tool operate in harsh environment, namely extreme conditions including high pressure from several hundred bars up to 2000 bars, high temperature up to 200° C., presence of corrosive fluids such as sulfuric acid, presence and contamination by solid particles such as scales, asphalthenes, sand particles, as well as multiphasic flow conditions (oil, gas, water). Further, there are also the space and power constraints associated to downhole tools deployment. Furthermore, there is the high shocks environment associated to wireline or drilling or production logging operations.